A luminescent phosphor compound is a compound that is capable of emitting detectable quantities of radiation in the infrared, visible, and/or ultraviolet spectrums upon excitation of the compound by an external energy source. A typical luminescent phosphor compound includes at least a host crystal lattice, an emitting ion (e.g., of a rare earth metal), and in some cases, a “sensitizing” ion (e.g., of a transition metal or of a different rare earth metal that can absorb and transfer the energy to the emitting rare earth metal ion). The production of radiation by a phosphor compound is accomplished by absorption of incident radiation by the emitting ion(s) or by either or both the host crystal lattice and the sensitizing ion(s), energy transfer from the host crystal lattice/sensitizing ion(s) to the emitting ion(s), and radiation of the transferred energy by the emitting ion(s).
The selected components of a phosphor compound cause the compound to have particular properties, including specific wavelengths for its excitation energy (“exciting radiation”), and specific spectral position(s) for peak(s) in energy emitted by the emitting ions of the phosphor compound (“emitted radiation”). Not every ion will have emission in all host crystal lattices. There are many examples in which radiation that has the potential for emission is quenched or the energy transfer from the absorbing ions or the host crystal lattice to the emitting ions is so poor that the radiation effects are barely observable. In other host crystal lattices, the radiation effects can be very large and with quantum efficiency near unity.
For a specific phosphor compound that does produce observable emitted radiation, the spectral position(s) of the peak(s) in its emitted radiation (i.e., its “spectral signature”) may be used to uniquely identify the phosphor compound from different compounds. Primarily, the spectral signature is due to the rare earth ion(s). However, spectral perturbations may be present due to the influence of the host crystal lattice on the various ions, typically through crystal field strength and splitting. This holds true for the temporal behavior of the emitted radiation, as well.
The unique spectral properties of some phosphor compounds make them well suited for use in authenticating or identifying articles of particular value or importance (e.g., banknotes, passports, biological samples, and so on). Accordingly, luminescent phosphor compounds with known spectral signatures have been incorporated into various types of articles to enhance the ability to detect forgeries or counterfeit copies of such articles, or to track and identify the articles. For example, luminescent phosphor compounds have been incorporated into various types of articles in the form of additives, coatings, and printed or otherwise applied authentication features.
An article that includes a luminescent phosphor compound may be authenticated using specially designed authentication equipment. More particularly, a manufacturer may incorporate a known phosphor compound into its “authentic” articles. Such a phosphor compound may be referred to as an “authenticating” phosphor compound (i.e., a phosphor compound having known spectral and possibly known temporal properties, as well as particular excitation conditions, which is used for identification and/or authentication purposes). Authentication equipment configured to detect the authenticity of such articles would have knowledge (e.g., stored information) of the wavelengths of absorbable exciting radiation and the spectral properties of emitted radiation associated with the authenticating phosphor compound. When provided with a sample article for authentication, the authentication equipment exposes the article to exciting radiation having wavelengths that correspond with the known wavelengths of absorption features of the luminescent phosphor that lead directly or indirectly to the desired emitted radiation. The authentication equipment senses and characterizes the spectral parameters for any emitted radiation that may be produced by the article. When the spectral signal of detected emitted radiation is within the authenticating parameter range of the detection apparatus that corresponds with the authenticating phosphor compound (referred to as the “detection parameter space”), the article may be considered authentic. Conversely, when the authentication equipment fails to sense signals expected within the detection parameter space, the article may be considered unauthentic (e.g., a forged or counterfeited article).
The above-described techniques are highly-effective at detecting and thwarting relatively unsophisticated forgery and counterfeiting activities. However, individuals with the appropriate resources and equipment may be able to reverse engineer an authentication system and/or to employ spectrometry techniques in order to determine the components of some phosphor compounds. The phosphor compounds may then be reproduced and applied to unauthentic articles, thus compromising the authentication benefits that may otherwise be provided by a particular phosphor compound. Accordingly, although a number of phosphor compounds have been developed to facilitate article authentication in the above-described manner, it is desirable to develop additional compounds and techniques for authenticating articles, which may render forgery and counterfeiting activities more difficult, and/or which may prove beneficial for identifying and tracking articles of particular interest. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.